38 THE VOYAGE OF H.M.S. CHALLENGER. 



annuli would completely fill up the gap, and that a continuance of subsequent growth 

 would restore the cu'cular figure. The specimen represented in fig. 7 is of peculiar 

 interest, as showing the early stage of this reparation ; to exhibit which more clearly the 

 specimen has been laid open by grinding it down towards its median plane. An irregidar 

 fracture has obviously been sustained along nearly half the margin of this disk, previously 

 to the formation of the last two concentric annuli ; and these annuli can be traced along 

 the entire length of the fractured margin (of which a portion is shown on a large scale in 

 fig. 7 a), just as along the unbroken periphery, — except that while the arrangement of the 

 chamberlets in these last two annuH is conformable to that of the annulus with whose 

 unbroken margin they are continuous, these chamberlets lie t«?iconformab]y along the 

 broken edge to those of the preformed structure. 



Relations to Simple Type. — We have now to consider the relations of the " complex " 

 plan of growi;h which is characteristic of Orbitolites complanata, to the " simple " plan 

 exhibited in Orbitolites tenuissima, Orbitolites mwginalis, and Orbitolites duplex ; and 

 have especially to inquire whether there is any evidence of the genetic derivation of the 

 higher type from either of the lower. 



In describing Orbitolites complanata, I have purposely limited myself to that typical 

 form which presents its characteristic features in their highest development, those features 

 being (l) the origin of thedisk in a large and thick "nucleus"; (2) the immediate 

 assumption of the cyclical plan of growth, as shown in the primal pullulation of 

 chamberlets round the whole periphery of the nucleus, so as to form a complete annulus ; 

 and (3) the immediate assumption of the complex plan of growth, shown in the separa- 

 tion, even in the very first annulus, of the two superficial layers by an intervening stratum, 

 as shown in vertical section in PI. VI. figs. 9, 10. But I find a considerable number of 

 disks, especially in the 1 8 fathoms' collection, which present that intermediate condition 

 on which I laid great stress in my former Memoir (§§ 57, 58), as indicating that the 

 " complex " type is only a more developed form of the " simple. " In such disks the 

 central portion is formed in every respect upon the "simple" plan, which afterwards 

 gives place to the " complex," sometimes rather suddenly, but generally more gradually, 

 at a variable distance from the centre. Such a " simple " condition may be inferred to 

 prevail in the interior part of any disk, whose peripheral portion is shown to be " complex " 

 by the multiple arrangement of its marginal pores, when its central portion is very thin, 

 its nucleus small, its first formed annuli not complete, and the form of its surface- 

 divisions circular tending to square, as in Orbitolites dupilex ; the passage to the complex 

 tjqDe being marked by the rapid thickening of the disk, and the narrowing of the 

 surface-divisions, so that they take-on the elongated form characteristic of the superficial 

 chamberlets oi Orbitolites complanata. This last change is well shown in PI. V. fig. 11, 

 which represents half of the sarcodic body of one of these sub-tjqaical forms ; the 

 sub-segments of the central portion of the disk (which are the summits of " simple " 



